A cylinder for an internal combustion engine may be constructed by boring an engine block or by inserting a liner (also called a sleeve) into a cylindrical space formed in an engine block. The following description presumes a cylinder with a liner construction; however the underlying principles apply as well to a bored construction.
A cylinder liner of an opposed-piston engine has a cylindrical inner wall that provides a bore with a longitudinal axis. Intake and exhaust ports are formed in the liner wall and located on respective sides of a central portion of the liner. Each port includes a plurality of port openings disposed in an annular array along a respective circumference of the liner, and adjacent openings are separated by solid portions of the liner wall called “bridges” or “bars”. (In some descriptions, each opening is referred to as a “port”; however, the construction of a circumferential array of such “ports” is no different than the port constructions described herein.) So constructed, the liner forms a “ported cylinder” when received in an opposed-piston engine.
When considering packaging in many applications, the length of a cylinder is one of the primary challenges of an opposed-piston engine. This is because there are two pistons coaxially disposed for opposed sliding motion in the bore between a top dead center location (hereinafter, “TDC”) and a bottom dead center location (hereinafter, “BDC”). Thus, the cylinder must be long enough to accommodate at least twice the length of each piston; in other words, the length of the cylinder is generally ≥4× the piston length. Any incremental reduction in these fundamental length limitations is therefore desirable when reduction in the engine profile is pursued.
Commonly-owned U.S. Pat. No. 8,935,998 describes a compact cylinder liner construction for an opposed-piston engine. As per a typical opposed-piston application including a ported liner, each piston in the cylinder is associated with a respective one of the two ports. In most applications, each piston has an upper ring pack adjacent the top land of the piston crown for containing combustion, and a lower ring pack in its lower skirt portion with which lubricant (engine oil) is scraped from the bore. Generally, the piston is somewhat longer than the longitudinal distance between the ring packs. When the piston is at TDC, the oil control (lower) ring pack is positioned near the outer edge of the port with which the piston is associated. The '998 patent describes a transition pattern in the bore diameter that permits an oil control ring pack to more closely approach the outer edge of the port when the piston is at TDC. This allows the length of the piston to be shortened, thereby leading to a reduction in the required cylinder length.
It is known that two-stroke cycle, opposed-piston engines provide superior power densities and brake thermal efficiencies as compared to their four-stroke counterparts. However, the length of the cylinder places a hurdle in the path of broad acceptance of opposed-piston technologies, especially in transportation applications where engine compartment space is limited. Accordingly, further reductions in cylinder length will extend the range of applications of opposed-piston technology.